Dual-band coupled VCO

ABSTRACT

In a dual band capable voltage controlled oscillator VCO circuit comprising two voltage controlled oscillator units VCO 1 , VCO 2 , the voltage controlled oscillator units VCO 1 , VCO 2  are synchronized and connected via at least two coupled transmission lines TL 1 , TL 2 , the transmission lines (TL 1 , TL 2 ) are arranged to operate according to one of two modes to enable varying a combined inductance of the synchronized oscillator units VCO 1 , VCO 2  and the oscillation frequency for the voltage controlled oscillator circuit VCO.

TECHNICAL FIELD

The present invention concerns voltage controlled oscillators ingeneral; especially a dual-band voltage controlled oscillator (VCO) withimproved phase noise performance

BACKGROUND

The development of wireless communication systems has increased thedemand for monolithically integrated, low-cost and low-phase-noisevoltage controlled oscillators (VCOs). At the same time, the developmentfor several communication standards utilizing different frequency bandshas pushed researchers to develop multi-band as well as multi-standtransceivers. This has, in turn, pushed researchers to look formulti-band VCOs, especially dual-band VCOs. One of the major demands ofsuch transceivers is the need for multi-band VCOs with good phase noiseperformance.

Known multi-band VCOs have been realized according to a variety ofapproaches. One known approach is to use switched capacitors orinductors in a LC resonator, as shown in FIG. 1 a-b [1], [2]. Theinductance or capacitance of the resonator is thus varied by utilizingswitchers V_(switch), consequently also the oscillation frequency-bandis varied. Alternatively, the total inductance of the resonator can bevaried by switching mutual inductance within a resonator, as shown inFIG. 2 and FIG. 3, [3], [4].

All the above described known solutions suffer from a common problem.The parasitic resistance of the switcher is it via actual switchers orswitched mutual inductance degrades the quality factor Q of theresonator. Consequently, the phase noise performance of the entire VCOis degraded. Even though increasing the physical size of the switchercan reduce the parasitic resistance to a certain extent, the largeparasitic capacitance results in a decrease of the tuning frequencyrange.

Yet another known approach has been to build two different frequencyVCOs in combination, and let the two VCO's share part of the resonator.By switching the bias current, one VCO works and the other stands byaccording to [5]. For example, as shown in FIG. 4, there are two VCOs.The resonator of VCO1 consists of one port of a transformer andcapacitors, while the resonator of VCO2 consists of two ports of thetransformer and capacitors. Thus, two voltage controlled oscillatorsVCO1, VCO2 have different oscillation frequencies, and they arecontrolled by a switched current bias. Another example is shown in FIG.5 [6], where either of the two voltage controlled oscillators VCO1, VCO2can be turned on by switching the respective bias current I_(d) andI_(c). Those two oscillators oscillate at a respective frequency, thus adual band VCO is obtained.

For the above described switched bias current, only one VCO works at anyone time, but the inactive or stand-by VCO is still connected to theresonator, consequently, the parasitic resistance and capacitance of theactive devices in the switched-off VCO has certain undesired effects onthe operating VCOs.

Due the above problems, there is a need for an improved dual bandvoltage controlled oscillator with reduced phase noise performance.

SUMMARY

An object of the present invention is to provide an improved dual-bandvoltage controlled oscillator.

A further object of the present invention is to provide a dual-bandvoltage controller oscillator with reduced phase noise performance.

According to a basic embodiment a dual band capable voltage controlledoscillator circuit VCO according to the invention comprises two voltagecontrolled oscillators VCO1, VCO2, wherein the voltage controlledoscillator units VCO1, VCO2 are synchronized and connected via at leasttwo coupled transmission lines TL1, TL2. The transmission lines TL1, TL2are arranged to operate according to either one of two modes to enablevarying a combined inductance of the synchronized oscillator units VCO1,VCO2 and thereby vary the oscillation frequency for the voltagecontrolled oscillator circuit VCO.

Advantages of the present invention comprise:

-   -   a dual band voltage controlled oscillator with improved phase        noise performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by referring to the following description takentogether with the accompanying drawings, in which:

FIG. 1 a is an illustration of a known reconfigurable dual-band voltagecontrolled oscillator;

FIG. 1 b is an illustration of another known reconfigurable dual-bandvoltage controlled oscillator;

FIG. 2 is an illustration of a known switched resonator;

FIG. 3 is an illustration of a further known switched resonator,

FIG. 4 is an illustration of a known dual-band VCO;

FIG. 5 is an illustration of another known dual-band VCO

FIG. 6 is an illustration of an embodiment of a dual band VCO accordingto the present invention;

FIG. 7 is an illustration of another embodiment of a dual band VCOaccording to the present invention;

FIG. 8 is an illustration of a cross-coupled dual-band VCO according toan embodiment of the present invention;

FIG. 9 a is an illustration of a Hartley VCO according to an embodimentof the present invention;

FIG. 9 b is an illustration of a Colpitts VCO according to an embodimentof the present invention;

FIG. 10 is an illustration of a differential dual-band Colpitts VCOaccording to an embodiment of the present invention;

FIG. 11 is an illustration of a dual-band VCO according to an embodimentof the present invention;

FIG. 12 a is a graph showing the frequency response from the negativeresistance;

FIG. 12 b illustrates a dual-band amplifier useable in combination withembodiments of the present invention;

FIG. 13 illustrates a dual-band cross-coupled VCO according to anembodiment of the present invention;

FIG. 14 a illustrates a dual-band Hartely VCO according to an embodimentof the present invention;

FIG. 14 b illustrates a dual-band Colpitts VCO according to anembodiment of the present invention;

FIG. 15 illustrates a differential dual-band Colpitts VCO according tothe present invention.

ABBREVIATIONS

CMOS Complementary Metal Oxide Semiconductor

IC Integrated Circuit

TL Transmission Line

VCO Voltage Controlled Oscillator

DETAILED DESCRIPTION

The present invention will be discussed in the context of a generalintegrated circuit scheme applicable to any communication technologyutilizing dual or even multiple frequency bands. The proposed VCOtopology can be implemented in any semiconductor technology, e.g., CMOS,bipolar, Silicon, GaAs etc. It can be fully integrated on a chip but canalso be made with discrete components or a mixture of ICs and discretecomponents.

The total inductance L_(i) (i=1,2) of a coupled transmission line pairis determined by the self-inductance L_(s,j) and by their mutualinductance M (see Appendix 1 for further description of mutualinductance and the dot convention). The transmission lines have twomodes of operation, namely even and odd mode. Accordingly, for operationin even mode, L_(i)=L_(s,j)+M; and operation in odd modeL_(i)=L_(s,j)−M. Consequently, by controlling the operation mode of thecoupled transmission lines TL, the total inductance can be varied;consequently, change the oscillation frequency of a coupled VCOsaccording to Equation 1.

$\begin{matrix}{f_{0} = \frac{1}{2\pi\sqrt{\left( {L_{s,i} \pm M} \right)C}}} & (1)\end{matrix}$

The present invention generally concerns a dual-band voltage controlledoscillator VCO based on two coupled voltage controlled oscillators VCO1,VCO2.

The two oscillators are synchronized via coupled transmission lines TL.By switching even or odd mode of the coupled transmission lines, theinductance of the LC resonator for each oscillator is varied, so is theoscillation frequency band for the combined voltage controlledoscillator. Furthermore, the coupled VCOs have a better phase noiseperformance than known single VCOs.

The present invention discloses various embodiments of a generalimproved dual band VCO comprising at least two coupled, synchronizedvoltage controlled oscillators VCO1, VCO2. The oscillators are coupledvia a mutual inductance coupled transmission line pair TL1, TL2,arranged to enable switching the operational mode of the coupledtransmission line pair TL1, TL2 to provide a dual mode of operation ofthe voltage controlled oscillator.

According to one embodiment, with reference to FIG. 6 and FIG. 7, theswitching action is enabled by means of switchers V_(switch) across thecoupled transmission line pair TL1, TL2. In the illustrated schematic,the respective voltage controlled oscillators VCO1 and VCO2 arerepresented by the negative resistor −R, and capacitor C, as well ascoupled transmission lines TL1, TL2 in the proposed VCO, Here, −Rrepresents the negative resistance provided by the power amplifier ofthe respective voltage controlled oscillators VCO1, VCO2. The switchersare located at the coupled transmission lines terminals (FIG. 6) or at acentral portion of the coupled transmission line pair (FIG. 7). Thepower amplifier preferably has sufficient gain to maintain the combinedVCO oscillating at both frequency bands, e.g. both for even and oddmode.

The thus described dual band VCO provides a voltage controlledoscillator with improved phase noise performance as compared to knownsolutions.

The provided switchers control the operating mode of the coupledtransmission line pair, namely, when the switcher is on, the coupledtransmission line pair TL1, TL2 operates at even mode f_(even) and themutual inductance M is positive.

Consequently, the coupled VCO works at low frequency band (see Equation1). Opposite, when the switcher is off, the coupled transmission linepair TL1, TL2 operates in odd mode f_(odd) and the mutual inductance Mis negative. Thus, the combined VCO works in a high frequency band.

By modifying the mutual inductance M of the coupled transmission linesTL1, TL2 the frequency difference between the two modes of operation isvaried. The mutual inductance M depends on the distance between the twocoupled transmission lines TL1, TL2, the length of the coupled part andthe location of the transmission lines. In other words, how far apartthe transmission lines are, how long the transmission lines are, and ifthe transmission lines are co-planar and/or stacked.

By utilizing the combination of two coupled VCOs and a mutual inductanceinduced transmission line pair, it is possible to vary the oscillationfrequency band of the entire voltage controlled oscillator, and tosynchronize the two single VCOs.

With reference to FIGS. 8, 9 a, 9 b, the concept of the combination ofsynchronized voltage controlled oscillators and coupled transmissionlines can be applied to various types of voltage controlled oscillatorsand combinations of such oscillators.

According to one embodiment, with reference to FIG. 8, two cross-coupledVCOs are synchronized and coupled via a mutual inductance transmissionline pair TL_(D). The switching of the operational mode of thetransmission line pair TL_(D) is enabled by means of two switchersV_(SWITCH) connecting respective first and second terminals of thetransmission lines TL1, TL2.

According to a further embodiment, with reference to FIG. 9 a, adual-band Hartley VCO comprises two Hartley voltage controlledoscillators coupled via two pairs of transmission lines TL_(D), TL_(g),each provided with a switch V_(switch).

Yet a further embodiment, with reference to FIG. 9 b, discloses adual-band

Colpitts VCO comprising two Colpitt voltage controlled oscillatorssynchronized and coupled via a coupled transmission line pair TL_(g).The operational mode of the transmission line pair is enabled using aswitch connected between first terminals of the transmission line pair.

In addition, but not shown, the present invention can be applied to adual band voltage controlled oscillator comprising at least two ClappVCOs. Because all the above VCOs have a LC resonator, the inductor or apart of an inductor can be coupled, to build a dual-band VCO.

With reference to FIG. 10, the general concept of the present inventioncan be used to construct differential VCOs, to provide differentialsignals regardless of the operational mode of the coupled transmissionlines. The example, as shown in FIG. 10, is based on two synchronizeddual-band Colpitts VCO according to the present invention, eachcomprising two synchronized Colpitt VCOs and a coupled transmission linepair TL_(g) and switches at said transmission lines. In this embodiment,transistors M1 and M2 always operate differentially and the differentialsignal can be provided at their base or emitter. It should be noted thatalso the cross-coupled dual-band VCO disclosed in FIG. 8 can be arrangedto deliver a differential signal.

According to a further embodiment and aspect of the present invention,it is equally possible to utilize the synchronized voltage controlledoscillators and the coupled transmission lines, but without providingactual switchers arranged at the transmission lines. Instead, the poweramplifier gain for each VCO is switched or varied between differentfrequency bands. Thus, the combined VCO can be forced to oscillate atonly one frequency band. Since at this oscillation frequency band onlythe VCOs loop gain can be larger than or equal to 1 to fulfill theoscillation condition. That frequency corresponds to one operationalmode of the coupled transmission lines. At another oscillation frequencyband or transmission line operational mode, the power gain is too smallto provide the VCO oscillation.

A simplified schematic of the proposed amplifier gain switching solutionis illustrated in FIG. 11. A dual-band amplifier is required for eachvoltage controlled oscillator VCO1, VCO2, in order to provide a powergain to maintain the combined VCO operating at one frequency band, asshown in the diagram in FIG. 12 a. In principle, any dual-band amplifiercan be used, provided that it can be switched. For example, in FIG. 12 ba switched dual-band amplifier is illustrated, where the switched sourcedegeneration is used to control the gain at different frequency bands.

The embodiment comprises parallel-connected inductor L_(s) and capacitorC_(s) (and the parasitic capacitor of the switcher M2) that constitute aresonator circuit which has a resonation frequency f_(even)

$\begin{matrix}{f_{even} = \frac{1}{2\pi\sqrt{\left( {L_{s,i} + M} \right)C}}} & (2)\end{matrix}$

Thus, when switcher M2 is on, the resonator has the largest impedance atf_(even) and causes the amplifier to have its lowest gain at f_(even).At high frequency, the resonator becomes capacitive with a quite lowimpedance, and consequently the amplifier has a high gain. In this case,the voltage controlled oscillator will operate at f_(odd), highfrequency band. Alternatively, when switching M2 off, the inductor Lsbecomes the source degeneration whose impedance increases withfrequency, therefore the amplifier exhibits high gain at f_(even) evenand low gain at f_(odd). In this case, the VCO will oscillate atf_(even) low frequency band.

Consequently, the dual band switching of the proposed VCO is realized byaltering the frequency response of the amplifier gain. In a similarmanner as the previously described switching embodiments, this can beapplied to different VCO topologies. Especially, the concept can beapplied to the same set up of synchronized voltage controlledoscillators as described with reference to FIGS. 8-10, but without theprovided switches at the transmission lines. Instead the switching isprovided via switched power amplifiers provided at the respective VCOs.With reference to FIG. 13, 14 a, 14 b, a dual-band cross coupled VCO(FIG. 13), a dual-band Hartley VCO (FIG. 14 a), a dual-band Colpitts VCO(FIG. 14 b), etc.

Finally, the proposed dual-band VCO can be used to constructdifferential VCOs, to generate differential signals for bothtransmission lines modes. As illustrated in FIG. 15, two dual-bandColpitts VCO are synchronized and the differential signal can be takenfrom the base or emitter of transistors M1 and M2. Moreover, thedual-band cross-coupled VCO in FIG. 13 can be utilized to providedifferential signals.

Advantages of the various embodiments of the present invention comprise:

-   -   When using the switcher(s) across the coupled transmission        lines, regardless if the switcher is either on or off, the        current through each switcher is very small, thus, the power        dissipation on the switcher is very small. This means the        degraded Q due to switcher can be ignored. This gives the        proposed VCO an advantage of better phase noise performance over        the dual-band VCOs using switched inductors or capacitors, or        switched mutual inductance. In the latter case, there is a large        ac current flowing through the switcher, when the switcher is        on. It can result in degraded Q of the resonator.    -   When using the dual-band switched power amplifier, the switcher        itself is not a part of VCO's resonator. Consequently, the VCO's        phase noise does not sensitive to the switcher's parasitic        resistance and capacitance.    -   In the proposed VCO, two coupled VCOs are synchronized, which        has of benefit to reducing the phase noise. In contrast, the        dual-band VCO utilizing switched bias [5], [6] has only one        working VCO at one time. The standing by VCO makes no        contribution to reducing phase noise, instead, the parasitic        resistance and capacitance of the active devices can reduce the        resonator Q, thus, increase phase noise.

It will be understood by those skilled in the art that variousmodifications and changes may be made to the present invention withoutdeparture from the scope thereof, which is defined by the appendedclaims.

REFERENCES

-   [1] A. Mazzanti, P. Uggetti, R. Battaglia, and F. Svelto, “Analysis    and design of a dual-band reconfigurable VCO”, Proc. Of the 2004    11^(th) IEEE Electronics, Circuits and Systems, pp. 37-41, 2004.-   [2] S. S. Broussev, T. A. Lehtonen, and N. T. Tchamov, “A wideband    low phase noise LC-VCO with programmable K_(VCO)”, IEEE Microwave    and Wireless Components Letters, 2007.-   [3] J. Cabanillas, “Coupled-inductor multi-band VCO”, patent number:    US2006/0033587 A1.-   [4] R. S. Kaltenecker, “Optimum RF VCO structure”, U.S. Pat. No.    6,943,635 B1.-   [5] A. Bevilacqua, F. P. Pavan, C. Sandner, A. Gerosa, and A.    Neviani, “Transformer-based dual-mode voltage-controlled    oscillators”, IEEE Trans. on Circuits and Systems-II, 2007-   [6] D. Back, J. Kim, and S. Hong, “A dual-band (13/22-GHz) VCO based    on resonant mode switching”, IEEE Microwave and Guided Wave Letters,    Vol. 13, No. 10, pp. 443-445, October, 2003-   [7] H. Jacobsson et al. “Very low phase-noise fully integrated    coupled VCOs”, in Proc. IEEE Radio Frequency Integrated Circuits    Symp. 2002, pp. 577-585.

APPENDIX 1

Mutual Inductance

Mutual inductance M is the concept that the current through one inductorcan induce a voltage in another nearby inductor. It is important as themechanism by which transformers work, but it can also cause couplingbetween conductors in a circuit.

The mutual inductance M is also a measure of the coupling between twoinductors. The mutual inductance by circuit i on circuit j is given bythe double integral Neumann formula

$M_{ij} = {\frac{\mu_{0}}{4\pi}◯{\int_{Ci}{\int_{Cj}{◯\frac{{\mathbb{d}s_{i}}{\mathbb{d}s_{j}}}{R_{ij}}}}}}$Dot Convention

In circuit analysis, the dot convention is used to denote the voltagepolarity of the mutual inductance of two components. (Reference is madeto FIG. 4 of this disclosure).

Two good ways to think about this convention:

1. The current goes into one dot (either dot) “tries” to come out of theother dot. “Into” meaning from the dot toward the inductor, andconversely “out” meaning from the inductor toward the dot.

2. Current going into a dotted terminal of the inductor induces apositive voltage at the other dot. Conversely, current leaving a dottedterminal induces a negative voltage at the other dot.

What is claimed is:
 1. A dual band capable voltage controlled oscillator(VCO) circuit, comprising: two voltage controlled oscillator units; andat least two transmission lines, wherein the two voltage controlledoscillator units are synchronized and coupled to each other via the atleast two transmission lines, the at least two transmission lines beingarranged to operate according to one of two modes to enable varying acombined inductance of the synchronized two oscillator units and anoscillation frequency of the VCO circuit.
 2. The circuit according toclaim 1, wherein the transmission lines are configured to operate at aneven mode or an odd mode.
 3. The circuit according to claim 1, furthercomprising at least one switching unit arranged at said transmissionlines to enable switching between said two modes.
 4. The circuitaccording to claim 1, further comprising two switching units arranged atand coupling a respective first terminal and second terminal of saidcoupled transmission lines.
 5. The circuit according to claim 3, whereinsaid at least one switching unit is arranged at and coupled to a centralportion of each of said two coupled transmission line.
 6. The circuitaccording to claim 1, further comprising switching units that controlpower gain of an amplifier in each voltage controlled oscillator.
 7. Thecircuit according to claim 6, wherein the power gain is configured tomaintain VCO oscillation at any one of said two modes of operation.
 8. Adual band capable voltage controlled oscillator (VCO) circuit,comprising: two voltage controlled oscillator units; and at least twocoupled transmission lines, wherein the two voltage controlledoscillator units are synchronized and coupled to each other via the atleast two coupled transmission lines, the at least two coupledtransmission lines being coupled to one another and arranged to operateaccording to one of two modes to enable varying a combined inductance ofthe synchronized two oscillator units and an oscillation frequency ofthe VCO circuit.